The present invention relates to a device for preventing the deposition of particles on a selected surface, and more particularly to a device for preventing the deposition of dust on a camera lens or transparent surface.
Lenses provide the interface between the interior of optical instruments and the exterior environment. The lenses of optical instruments operating in dusty environments soon become covered with a layer of dust which hinders or disturbs the passage of light into the instrument, thus distorting or interfering with the optical signal or image received by the instrument. Lenses in such environments must therefore be cleaned frequently, but it is often not practical to do so. For example, where optical instruments are used to monitor certain phases in the automatic mass-production of goods, it may be necessary to stop production to clean the lenses. Such stoppage leads to loss of production and may lead to waste during the start-up or stopping of the mass-production machine.
One known technique for preventing dust settlement concerns the use of a cylindrical dust shield in which air is projected at a right angle to the plane of the lens forming a turbulent air flow across the lens. This technique is not entirely effective, and unacceptable amounts of dust coat the lens after about one hour in a heavy dust environment.
U.S. Pat. No. 4,240,691 refers to preventing dust settlement on a lens by placing the lens inside a tube and by introducing a laminar gas flow in the tube away from the lens. One problem with this technique is that the tube needs to be very long in relation to its diameter in order to obtain a laminar flow sufficient to minimize dust accumulation. This causes the field of view of the lens to be restricted.
Another known attempt at preventing dust deposition concerns placing the lens in a porous cylinder and raising the air pressure outside the cylinder so that air passes through the cylinder, is widely dispersed by the cylinder, and flows away from the lens. However, this technique is not satisfactory because high-velocity turbulent jets may develop and nullify the dust prevention process. These high-velocity turbulent jets may develop because of the potentially conflicting requirements of a laminar air flow which is both uniformly widely dispersed and ample. Widely dispersed laminar flow requires the use of a cylinder with very low porosity, and ample flow through the cylinder requires a large air pressure difference across it. Thus, there is a relatively high pressure gradient from the exterior to the interior of the cylinder, and any minor leakage regions that may be present will allow the air to escape rapidly. These leakage regions could arise if the cylinder is slightly damaged, non-uniform, or poorly seated.
Applicants have determined that the porous cylinder technique may be improved, and jets may be eliminated, by using a two-stage gas distribution system feeding a highly-porous cylinder. In this system, a first plenum having an annular outlet is used to reduce the pressure of the gas and to distribute it widely into a second plenum which, in turn, supplies the gas to the exterior of the cylinder. In this way, only a small pressure gradient exists from the exterior to the interior of the cylinder, and minor leaks do not affect the flow substantially. However, this technique still relies on a relatively long porous cylinder that unduly limits the field of view of the lens.
Accordingly, there is a continuing need for improvements in devices and methods for minimizing the deposition of dust on optical lenses having a broad range of fields of view, and in devices for maintaining such lenses sufficiently dust free for extended operation. There is also a need to provide such a device with a laminar gas flow without restricting the field of view of the lens.